Electrophoretic displays are well known in the art. An electrophoretic display generally comprises a suspension of one or two charged pigment particles colloidally dispersed in a clear or colored liquid of matching specific gravity and contained in a cell comprising two parallel and transparent conducting electrode panels. The charged particles are transported between the electrode panels under the influence of an electric field, and can therefore be made to display an image through appropriate application of the electric field on the electrodes. The advantages of electrophoretic displays as a means for providing information and displaying images has been well appreciated.
U.S. Pat. No. 4,272,596, incorporated herein by reference in its entirety, illustrates an electrophoretic display structure. The electrophoretic display device comprises a white marking material such as titanium dioxide suspended in a colloidal dispersion containing black colorant such as colloidal size iron oxide particles known as ferrofluids. Image formation is achieved electrophoretically by selective application of an electric field across the imaging suspension. In particular, a pair of electrodes associated so as to form a cavity therebetween, which cavity is filled with the aforementioned suspension medium. A source of electrical potential is coupled to the electrodes and when an electric field is applied, the marking particles form an image as they follow the field.
U.S. Pat. No. 6,113,810, incorporated herein by reference in its entirety, describes a dielectric dispersion for use in an electrophoretic display that includes a dielectric fluid, a first plurality of particles of a first color having a surface charge of a selected polarity dispersed within the dielectric fluid and a second plurality of particles of a second color having a surface charge of opposite polarity to that of the first plurality and a steric repulsion thereto preventing coagulation of the first and second plurality of particles. Each set of particles is formed with unique secondary and functional monomers. Corresponding charge control agents are added to the dispersion to establish opposite polarities on the respective particles.
U.S. Pat. No. 6,017,584, incorporated herein by reference in its entirety, discloses electrophoretic displays and materials useful in fabricating such displays. In particular, encapsulated displays are disclosed in which particles encapsulated therein are dispersed within a suspending, or electrophoretic, fluid. This fluid may be a mixture of two or more fluids or may be a single fluid. The displays may further comprise particles dispersed in a suspending fluid, wherein the particles contain a liquid. In either case, the suspending fluid may have a density or refractive index substantially matched to that of the particles dispersed therein. Application of electric fields to the electrophoretic displays affects an optical property of the display.
U.S. Pat. No. 6,577,433, incorporated herein by reference in its entirety discloses an electrophoretic display liquid composition for use in an electrophoretic display device that has a multiplicity of individual reservoirs, each containing the display liquid of two sets of particles dispersed in a transparent liquid system as well as at least one charge director dissolved or dispersed in the liquid system, or physically embedded on the surface of the particles or chemically bonded on the surface of the surface of the particles, the two sets of particles exhibiting different, contrasting color and different charging properties from each other. The charge director(s) may include a metal salicylate compound. The particles may be modified with charge controlling agents, and may also include a set of magnetic particles. The transparent liquid system may include two immiscible liquids having different densities with the sets of particles having densities in between the densities of the two immiscible liquids such that the particles rest at an interface between the two immiscible liquids.
U.S. Pat. No. 6,525,866, incorporated herein by reference in its entirety, discloses an electrophoretic display liquid composition for use in an electrophoretic display device that has a multiplicity of individual reservoirs, each containing the display liquid of at least two sets of particles dispersed in a transparent liquid system, the at least two sets of particles exhibiting different, contrasting color and different charging properties from each other, and at least one of the sets of particles containing flow aid particles as additives upon an external surface of the particles. Preferred flow aid additives include silica and titania particles.
Electrophoretic display is thus based on the migration of charged particles suspended in an insulating fluid under the influence of an electric field. The particles used in such displays to date have been charged by adding a charge control agent, which is capable of ionic dissociation, to the dielectric fluid during preparation of the non-aqueous display dispersion. Examples of charge control agents used have included bis-(2-ethyl hexyl) sodium sulfosuccinate and basic barium petronate (BBP). Dissociation of the charge control agent into positive and negative ionic species in the dielectric fluid results in preferential surface absorption of ions of one polarity by the particles. The particles therefore become charged. The resulting dispersion contains a complex mixture of particles including charged particles, excess free ions and counter-ions. Due to the presence of excess free ions, such electrophoretic display inks are characterized by high electrical conductivity, Conductivity has been shown to increase with concentration of the added charge control agent, and is typically 100-1000 times higher compared to the dielectric fluid. High conductivity of the ink results in increased power consumption and slower switching speed of the display.
While known electrophoretic display devices, compositions and processes for displaying images with such known devices are suitable for their intended purposes, a need remains for an electrophoretic display that remains stable for long periods of time and that reliably and rapidly displays and/or changes an image, and in particular a full color image.